Phylogeny and ontogeny of the habenular structure

Abstract

Habenula is an epithalamic nucleus connecting the forebrain with the ventral midbrain and hindbrain that plays a pivotal role in decision making by regulating dopaminergic and serotonergic activities. Intriguingly, habenula has also attracted interest as a model for brain asymmetry, since many vertebrates show left-right differences in habenula size and neural circuitry. Despite the functional significance of this nucleus, few studies have addressed the molecular mechanisms underlying habenular development. Mammalian habenula consists of the medial and lateral habenulae, which have distinct neural connectivity. The habenula shows phylogenetic conservation from fish to human, and studies using genetically accessible model animals have provided molecular insights into the developmental mechanisms of the habenula. The results suggest that development of the habenular asymmetry is mediated by differential regulation of the neurogenetic period for generating specific neuronal subtypes. Since the orientation and size ratio of the medial and lateral habenulae differ across species, the evolution of those subregions within the habenula may also reflect changes in neurogenesis duration for each habenular subdivision according to the evolutionary process.

Keywords: brain asymmetry; evolution; habenula; interpeduncular nucleus; lateralization; monoamines; neurogenesis; zebrafish.

Figure 1

Evolutionary conservation of the medial and lateral habenular pathways in vertebrates. Schematic diagram of sagittal sections from rat (A) and zebrafish (B) showing homologs for the medial (red) and lateral (blue) habenular circuitries. The entopeduncular nucleus [purple in (B)] sends axons to the habenula in zebrafish, although the target of those axons within the habenula remains unclear. RMTg, rostromedial tegmental nucleus; SNc, substantia nigra, pars compacta; VTA, ventral tegmental area.

Figure 2

Subnuclear organization of the habenula in vertebrates. (A–E) Schematic drawings of coronal sections taken at the level of the habenula, showing regions homologous to the mammalian medial (light green) and lateral habenulae (pink) of the macaque monkey (A), rat (B), side-blotched lizard (C), European frog (D), zebrafish (E). The small dots indicate regions rich in cell bodies, and the remaining gaps are neuropils. (A,B) were adapted from Amo et al. (2010). (C,D) Schematics modified from data presented previously by Guglielmotti and Fiorino (1998) (D) and Engbretson et al. (1981) (C). (E) Schematic drawing based on a coronal section of adult zebrafish stained with cresyl violet. (F) Development of the ventral habenula, based on expression patterns of the ventral habenular marker, diamine oxidase. Coronal views of Tg(brn3a-hsp70:GFP) zebrafish showing the distributions of dao-expressing cells in the ventral habenula (red) and GFP-expressing cells in the medial subnucleus of the dorsal habenula (green). Developmental stages are indicated beneath each panel. A section of adult zebrafish habenula (rightmost panel) was counter-stained by 4′,6′-diamidino-2-phenylindole (DAPI; blue). dpf, Days postfertilization.

Figure 3

Developmental mechanism of the habenula in genetically accessible model animals. (A) Schematic diagram of a sagittal section from E14 rat showing the habenular primordium (red), developing axons from the habenula (fasciculus retroflexus, red arrow) extending in between prosomere 1 (p1) and prosomere 2 (p2), and developing afferents to the habenula (stria medullaris) running along the dorsolateral surface of prosomere 3 (p3) and p2 (as blue arrow). The dorsal part of p2 and p3 expressed Fgf8 (light blue), which drives essential signaling for habenular development. Areas expressing guidance molecules for the habenular axons are designated by purple (Sema3F) and light green (Netrin1). AEP, anterior entopeduncular area; LGE, lateral ganglionic eminence; MGE, medial ganglionic eminence. The rostral and dorsal side is oriented at the left and top of the panel, respectively. (B) Neurogenetic model for the generation of left–right differences in the zebrafish dorsal habenula. At 18–24 h postfertilization (hpf), Nodal, and its downstream genes (green in the top panel showing dorsal view of Tg[lefty1:GFP]) are activated in the left dorsal diencephalon. Unilateral activation of Nodal signaling in the dorsal diencephalon directs the parapineal migration toward left (black arrow in the middle panel) and biases early neurogenesis in the habenular primordium with anti-symmetric nature in the presence of Fgf8 (red arrow between top and middle panels). Under those influences, common neural stem cells (light blue in the bottom panel) in the habenular primordium start to generate neural precursors for the lateral subnucleus (dHbl, red in the bottom panel) predominantly on the left side, under the influence of reduced Notch activity, and then neural precursors for the medial subnucleus (dHbm, green in the bottom panel) predominantly on the right side during late stages of neurogenesis peaked at 48 hpf. PO, pineal organ; PP, parapineal organ.